Experimental autoimmune encephalomyelitis (EAE) is an autoimmune demyelinating disease of the central nervous system (CNS) induced by CD4+ T cells reactive with myelin antigens, including myelin basic protein (MBP). It is widely used as an animal model of multiple sclerosis (MS). There is growing evidence for a role of the Th1 polarizing monokine, IL-12, in the pathogenesis of EAE as well as MS. We have recently found that primed MBP-reactive CD4+ Th1 cells are prevented from inducing disease in naive syngeneic recipients when coinjected with a neutralizing antibody against IL-12. Spinal cords from the protected mice are free of infiltrates. In this proposal we plan to expand upon these findings to elucidate the mechanism of action of anti-IL-12 in suppressing CNS inflammation. To do so, we will use an adoptive transfer protocol in which donor T cells can be identified since they express a congenic Thy marker. The goal of Aim 1 is to distinguish between two hypotheses: (i) anti- IL-12 blocks the passage of effector T cells across the blood- brain-barrier; or (ii) anti-IL-12 triggers the premature death of effector T cells. Based on the results of preliminary experiments, we will measure the effects of IL-12 on the expression of candidate adhesion and chemotactic molecules or pro- and anti-apoptotic mediators. In Aim2 we will assess the efficacy of anti-IL-12 therapy when initiated following the onset of clinical signs. These experiments will test the hypothesis that autoimmune effector cells depend on IL-12 to maintain their biological activities relatively late in the pathogenic process. We will assess whether anti-IL-12 suppresses the production of myelinotoxic molecules, such as TNFalpha or Lymphotoxinalpha, or induces the production of immunosuppressive factors, such as IL- 10 or TGFbeta, in the CNS. Furthermore, we will determine whether IL-12 neutralization blocks determinant spreading in the adoptive transfer recipients, thereby suppressing future relapses. These studies may have therapeutic relevance to MS as well as provide insights into the pathways by which IL-12 regulates differentiated effector T cells.